A. Field of the Invention
The invention generally concerns the use of a catalyst capable of catalyzing a dry reformation of methane reaction. The catalyst has a core-shell structure with an active metal deposited on the surface of the shell. The shell has a redox-metal oxide phase that includes a metal dopant incorporated into the lattice framework of the redox-metal oxide phase.
B. Description of Related Art
Synthesis gas or “syngas” is a gas mixture that includes carbon monoxide and hydrogen. Syngas is typically used as an intermediary gas to produce a wide range of various products, such as mixed alcohols, hydrogen, ammonia, i-C4 hydrocarbons, mixed alcohols, Fischer-Tropsch products (e.g., waxes, diesel fuels, olefins, gasoline, etc.) methanol, ethanol, aldehydes, alcohols, dimethoxy ethane, methyl tert-butyl ether, acetic acid, gas-to-liquids, butryaldehyde, etc. Syngas can also be used as a direct fuel source, such as for internal combustible engines.
One of the more common methods of producing syngas is by oxidizing hydrocarbon gases such as methane. For instance, the controlled oxidation of methane can be carried out using carbon dioxide, water, oxygen, or a combination of such materials. For industrial scale applications, methane can be reformed into syngas by using steam, as shown in the following reaction:CH4+H2O→CO+3H2 The ratio of CO/H2 obtained in steam reforming process is about 0.33. Many applications, however, require a CO/H2 of about 1.0. Such applications include production of aldehydes, alcohols, acetic anhydride, acetic acid, ethers, and ammonia. Therefore, the current solution is to remove excess H2 from the produced syngas using separation techniques, which can decrease efficient production while simultaneously increasing associated costs. The ratio of CO/H2 may be increased to about 1.0 by dry reforming of methane. In dry reforming of methane, methane is reacted with carbon dioxide or a mixture of carbon dioxide and oxygen as shown in the following equations:CH4+CO2→2CO+2H2 2CH4±CO2+O2→3CO+3H2+H2O
Catalysts are used to increase the rate of the reaction for both of the above reforming reactions. Supported or bulk catalyst containing Group VIII (Columns 8-10) metals are catalytically active towards reforming reactions. By way of example, Ni based catalysts can be used in steam reforming and dry methane reforming, however, the reaction condition for methane dry reforming can be more severe than that of methane steam reforming due to application of high H2O/CH4 ratio in methane steam reforming in comparison with CO2/CH4 ratio in dry reforming reaction. Several studies have shown that the nature of support employed influences the catalytic activity. One problem associated with dry reforming (using carbon dioxide) of methane is that current catalysts are prone to sintering, which reduces the active surface of the catalyst. Other problems associated with steam reforming and dry methane reforming reactions include growth of carbon residuals (e.g., encapsulating carbon, amorphous carbon, carbon whisker, filamentous carbon, and graphite) on the surface of the supported catalyst. Carbon growth can lead to deactivation of the catalyst due to blockage of catalytic sites (e.g., metal sites), degradation of the catalyst, reactor plugging or combinations thereof.
Several recent disclosures have focused on to improving the activity and life of reforming catalysts by attempting to control the size of the particle deposited on the surface of the support. By way of example U.S. Patent Publication No. 2014/0097387 to Biausque et al. discloses the synthesis of nickel-platinum (NiPt) nanoparticles and depositing the nanoparticles on an alumina, silica, titania or activated carbon support. U.S. Patent Publication No. 2014/0005042 to Feaviour describes a method of making a steam reforming catalyst that includes spraying a support with catalytic metals. Other disclosures have focused on methods to improving the support for various catalytic materials. For example, U.S. Patent Application No. 2014/0332726 to D′ Souza et al. describes synthesis of La2Zr0.88Rh0.12O7 pyrochlore catalysts grafted on a bimetal oxide support (e.g., MgAl2O4 support).
Even further, the cost of rare earth or noble metals used in the catalysts can be significant. Further, the associated methods for preparing such catalysts can be inefficient and can suffer from scalability for commercial manufacturing processes. Still further, many of the currently available catalysts suffer from mechanical strength issues and can mechanically break down during use.
One approach to improving the efficiency of reforming reactions is to develop catalysts that can catalyze a combination steam reforming and carbon dioxide reforming reaction (e.g., CH4/CO2/H2O). By way of example, Ni based catalysts can be used in steam reforming and dry methane reforming, however, the reaction conditions for methane dry reforming can be more severe than that of methane steam reforming due to application of high H2O/CH4 ratio in methane steam reforming in comparison with CO2/CH4 ratio in dry reforming reaction. In yet another example, U.S. Pat. No. 8,729,141 to Bae et al., and U.S. Pat. No. 8,524,119 to Jun et al. disclose a Ni/Ce/MgAlOx, or Ni/Ce—Zr/MgAlOx catalyst for the combined reforming of natural gas and carbon dioxide. U.S. Patent Application Publication No. 2014/0148332 to Moon et al. describes a bi-catalyst for the combination of steam and dry reforming of methane (e.g., CH4/CO2/H2O) that includes a mixture of Ni/MgO/Al2O3 (catalyst 1) and metal oxide catalyst (catalyst 2) that includes magnesia, nickel, vanadium, tungsten, iron, molybdenum or chromium. Jun et al. (“Kinetics modeling for the mixed reforming of methane over Ni—CeO2/MgAl2O4 catalyst”, Journal of Natural Gas Chemistry, 2011, Vol. 20, pp. 9-17) describes the modeling of a Ni—CeO2/MgAl2O4 catalyst in a combined steam and dry methane reforming reaction, which excluded the parameter of catalyst deactivation by coking. These catalysts suffer in that they are prone to carbon growth at high pressures.